Not much scares the archetypal Real Man. Changing the first nappy, perhaps. That bit in Audition. Having to watch Loose Women every day for the rest of their lives. And, of course, the heebie-jeebie-inducer that is European football's away goals rule.

It is difficult to think of anything, certainly in sport, that provokes quite such an irrational dread, and it will surely be demonstrated again by the asphyxiating nervousness among Manchester United fans (and, probably, players) in the early stages of tonight's match against Barcelona. For one night only, they should replace the Champions League theme tune with the music from Jaws.

Or maybe John Goodman could rock up, because there will be no greater demonstration of the life of the mind. The Barça manager Frank Rijkaard summed things up perfectly when he said: "They have more to think about than we do." And in the visceral world of sport, binge-thinking is never to be encouraged.

Ahead of the second leg, a 0-0 draw often serves to stifle the home team and embolden the away side. That's because, in the life of the mind, the away goal is king, a gleaming jewel in the desert sun. It is equally precious to the neutral: the concept adds such danger and psychological complexity to a familiar format - as well as prompting people to lose it on live TV - that, along with the iTunes sampler and speaker pillows, it deserves to make a list of the Best Ideas Ever.

Its seductiveness is such that many feel a 2-1 defeat away from home in the first leg is preferable to a 0-0 draw, even though this is a statistical contradiction. Increasingly, a 0-0 draw is perceived as a good result for the home side, and in some successful cases has arguably been actively sought by the home side. Just ask the Rangers manager, Walter Smith.

Admittedly there are reasons for Manchester United fans to be more fearful than most. The precedent monkey has built a house on their back: under Sir Alex Ferguson they have lost their three most recent European ties in which they drew the first leg 0-0 away from home, to Rotor Volgograd, Monaco and Real Madrid. Any concern that tonight will be fourth time unlucky is likely to be increased by the fact that the barely fathomable conservatism of their ostentatious managerial duo should ensure Barcelona have at least 60% of possession, enough to have even the corporate bums squeaking. And if United had shown even a smidgen of ambition in the Nou Camp last week rather than playing 9-0-1, they might well have nailed the tie already.

Had United drawn 1-1 in the Camp Nou, the smell of fear would be emanating from only a select few. Yet the difference between 0-0 and 1-1 is minimal, and only amounts to two results (0-0 and 1-1) in the second leg. Logically, 0-0 should be an excellent result. It pares the tie down perfectly: if you win at home, you go through. In the case of United, that home advantage is significant enough for them to have set a Champions League record of 11 straight wins. Indeed, since the inception of the competition, 63% of sides playing the second leg at home after a 0-0 first-leg draw have gone through. A 13% improvement, give or take, in the chances of progression seems like a good deal.

It is not, of course, quite as simple as that. While 0-0 is a better first-leg result than 1-2, it prompts a subtle and possibly damaging shift in psychology: there is a huge difference in needing to score to win the tie (as with 0-0) and having to score to avoid defeat (as with 1-2 when, almost as a happy bonus, a goal would also win the tie).

Indeed in the Champions League, a greater percentage of sides (66 to 63) have gone through after a 2-1 away defeat than after a 0-0 draw. (The percentage is identical for a 1-1 and a 2-1 defeat.) Perhaps the fear stems from the fact that one of football's most comforting certainties, the middle ground, is taken out of the equation: the draw is impossible once an away goal has been scored following a 0-0, so every goal effects a decisive swing.

That presents the home side with the ultimate stick-or-twist dilemma; it is a test of quality but, more importantly, of courage, because 0-0 is the only scoreline that represents parity at the start of the second leg. And while, in the circumstances, a counter-attacking approach from the away side has been legitimated by 50 years of precedent, the home manager is under pressure to flex his muscles and flash his cojones: to actively try to win the game rather than not lose it. For the cautious beast that is the modern manager, that might be the scariest thing of all.